Sorry for delay in responding, been running some more tests .......
No linedrops overnight Fri/Sat with SNRM stabilized at 18-20 & speed c.30Mb/s.
I raised the issue with ISP who wanted all the usual actions, e.g. reboot, check cables, etc, etc.
Before passing to Openreach, they asked me to try using the NTE5C test socket.
I replaced the existing Cat5e hardwired connection with an old RJ11 shielded modem cable and, wow, SNRM 6dB, speed 56Mb/s.
Connection has been rock solid over last 24 hrs.
So now I will investigate whether it's the Cat5e connection or something on the internal phone line that's the culprit.
Thanks for your assistance.